1. Field of the Invention
The present invention relates to a method for selective growth of silicon epitaxial films utilizing insulating film masks and silane (SiH.sub.4) or silane-based gases such as disilane (Si.sub.2 H.sub.6), diborane (B.sub.2 H.sub.6), germane (GeH.sub.4) or the like as raw material gases and particularly to a method for controlling a shape of an end portion of the selectively grown silicon epitaxial film in contact with a sidewall of an insulating film mask.
2. Description of the Related Art
First, an apparatus used in the growth method of this kind will be outlined.
FIG. 9 shows an example of an apparatus used for selective growth of silicon epitaxial films on a silicon substrate utilizing silane or silane-based gases such as disilane or the like as a raw material.
A selective silicon epitaxial growth apparatus of this kind is provided, in general, with a vacuum chamber consisting of a growth chamber 2 and a heater chamber 3 equipped with a substrate heater 4, a susceptor 5 for supporting a silicon substrate 1 and a turbo molecular pump 6 for differential pumping of the growth chamber 2 and the heater chamber 3. As shown in FIG. 9, the growth chamber 2 and the heater chamber 3 are separated by means of a silicon substrate 1 on which the selective growth is to be performed. Silane-based raw material gases, for example, silane, disilane, or diborane or germane are introduced into the growth chamber 2 from a gas nozzle 9 provided at the upper portion of the growth chamber 2. In this apparatus, only the silicon substrate 1 is heated by the substrate heater 4, so that the stainless steel growth chamber 2 is not heated. Therefore, the raw material gases introduced from the gas nozzle 9 are not decomposed in the vapor phase but are thermally dissociated and adsorbed on silicon atoms of the silicon substrate 1. There, they release extra hydrogen atoms todeposit silicon epitaxially.
On the other hand, since the raw material gas directly reaches the surface of the silicon substrate 1 without producing chemically active intermediate products in the vapor phase, the raw material gas molecules are not easily adsorbed on masks made of insulating films such as silicon oxide or silicon nitride. In the alternative, it is difficult for silicon to grow, because even if the raw material is adsorbed on the silicon surface, it is immediately desorbed. Consequently, silicon grows selectively on the silicon substrate during a latent period until formation of silicon is initiated on the mask film and such latent period or "dead time" is determined by the substrate temperature and a flow rate of the raw material gas.
In such selective epitaxial growth processes of silicon, under certain growth conditions, "facetting" or formation of crystalline facets occur near the end portions of the selectively grown film where they are in contact with the sidewalls of the insulating film masks. Due to this facetting, the thickness of the selectively grown silicon film is decreased at such end portions of the epitaxial films. As described in the Journal of Crystal Growth 136 (1994) pp. 349-354, facets have different surface free energies depending on the surface orientation of the silicon crystal, and therefore not only the (100) surface, but also (111) or (311) surfaces having a small surface free energy are also formed at the initial stage of silicon growth on, for example, the (100) silicon substrate. Facetting occurs as a result of the epitaxial growth progressing with this shape or condition intact. The reason why a surface having a small surface free energy is formed as described above is considered to be that increase of free energy due to the precipitation of crystal silicon is minimized by formation of such facets.
As described, for example, in the official gazette of the Japanese Patent Laid-Open No. Hei 4-74415(1992), facet formation is suppressed at the end portion of the selectively grown silicon epitaxial film, i.e., the area in contact with the insulating film mask sidewall, if the growth condition is such that the substrate temperature is set low and the flow rate of raw material gas is set high. When the substrate temperature is set low and flow rate of raw material gas, for example, of disilane is set at a high value, facetting becomes difficult for the following reasons. At lower substrate temperatures, hydrogen atoms dissociated from disilane are adsorbed more easily on the silicon surfaces. The amount of adsorption also increases when the flow rate of disilane is higher. When hydrogen terminates the silicon atoms on the surface, the surface free energy at the silicon surface terminated by adsorbed hydrogen atoms is remarkably lowered and the resulting dependence of surface free energy on the surface orientation can be eliminated, thereby suppressing facetting.
On the contrary, when the substrate temperature is set high and the flow rate of raw material gas, for example, disilane is set low, facetting is enhanced because hydrogen adsorption is suppressed and the effect of reducing dependence of surface free energy on surface orientation due to hydrogen termination becomes smaller.
An important application of the selective epitaxial silicon film is formation of a shallow junction structure at the source/drain portion of a very small MOS transistor of the 0.1 .mu.m rule level. In this kind of MOS transistor, after device area isolation oxide films and gate electrodes are formed on the silicon substrate, epitaxial silicon films are selectively grown on the portions of silicon which will become source/drain regions in order to form a source/drain structure having a shallow PN junction. This is done to suppress the so called "short channel effect". Thereafter, ion implantation is performed to form the source/drain region in a conventional manner. However, if a facet is formed at the end portions of the epitaxial film, as explained previously, depth of the implanted PN junction increases at portions close to the sidewall area where the facet is formed because there, the epitaxial film is thinner. As a result, the desired effect of suppressing the short channel effect is deteriorated.
Because of the reason explained above, it has been long desired to provide a method for selective epitaxial growth of silicon by which the facet is never formed at the sidewall portion. In the official gazette of the Japanese Patent Laid-Open No. Hei 4-74415(1992) explained above, a conventional method for selective epitaxial growth of silicon that focuses on the fact that formation of the facet depends on difference of the growth condition is disclosed. There, the growth condition that the substrate temperature is set low and flow rate of the raw material gas is set high is introduced as the method for growth of selective epitaxial film by which a facet is never formed.
The above conventional method for selective epitaxial growth of silicon has certain drawbacks which will be explained hereafter with reference to FIG. 6. FIG. 6 shows, for 4 different growth temperatures, maximum attainable selective epitaxial film thicknesses as a function of disilane flow rate, wherein selective growth is performed in an opening formed on an oxide film mask using disilane as a raw material gas. It is obvious from FIG. 6 that the thickness of a silicon epitaxial film which may be selectively grown under the conditions for suppressing facetting, i.e., low substrate temperature and high flow rate of raw material gas, becomes very small. This poses a problem when application of the selective epitaxial silicon film to the shallow junction source/drain region of the MOS transistor explained above is attempted, because a thickness of about 1000 .ANG. is required for the epitaxial film. Therefore, in order to selectively form a silicon epitaxial film of about 1000 .ANG., growth must be performed under the growth condition of substrate temperatures higher than 650.degree. C. and flow rate of disilane lower than 4 sccm. This growth condition inevitably includes a problem that facets are formed at the end portions of the selective silicon epitaxial film as described previously.